Process for making improved strength dental amalgam

ABSTRACT

A closed container having therein a mass of a particulate alloy containing silver and tin for use in making dental amalgam, and an inert atmosphere in the container. The container can be an enclosed capsule in which the liquid mercury is in a separate enclosed rupturable chamber inside of the capsule which keeps the mercury out of contact with the alloy until the chamber is ruptured. A method of making a dental amalgam by combining mercury and a particulate dental alloy, used in making dental amalgam, in an inert atmosphere.

United States Patent [1 1 Hansen [54] PROCESS FOR MAKING IMPROVEDSTRENGTH DENTAL AMALGAM [75] Inventor: David A. Hansen, Columbia, Mo.

[73] Assignee: The Curators of the University of Missouri, Columbia, Mo.

[22] Filed: May 20, 1974 [21] Appl. No.: 471,490

Related U.S. Application Data [62] Division of Ser. No. 264,619, June20, 1972, Pat. No.

[52] U.S. Cl 75/169; 75/173 R; 75/135 [51] Int. Cl. C22c 7/00 [58] Fieldof Search.. 75/169, 173 R, 173 A, 173 C, 75/135; 259/DIG. 20; 206/219,220, 221

[56] References Cited UNITED STATES PATENTS 3,141,761 7/1964 Rohm 75/169X FOREIGN PATENTS OR APPLICATIONS 1,242,585 8/1971 United Kingdom 75/l69June 17, 1975 Primary Examiner-L. Dewayne Rutledge Assistant ExaminerM.J. Andrews Attorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose 57 ABSTRACT A method of making a dental amalgam by combining mercury anda particulate dental alloy, used in making dental amalgam, in an inertatmosphere.

3 Claims, No Drawings PROCESS FOR MAKING IMPROVED STRENGTH DENTALAMALGAM This is a division of application Ser. No. 264,619, filed June20, 1972 now U.S. Pat. No. 3,841,467.

This invention relates to metal alloys. More particularly, thisinvention is concerned with improvements in dental amalgams for teethfillings.

in filling a tooth cavity with a so-called silver filling, the dentistuses an amalgam formed by combining liquid mercury with a powdered orgranular alloy. The alloy is largely silver and tin. To meet thestandards of the American Dental Association (American DentalAssociation Specification No. 1 for alloy for dental amalgam, pages132l35 of Guide to Dental Materials and Devices, 5th Ed, l970l97l;published by the American Dental Association.) the alloy must be atleast 65% silver and must not contain more than 29% tin, 6% copper, 2%zinc and 3% mercury. The amalgam is prepared by combining about equalamounts by weight of liquid mercury and powdered alloy.

Dental amalgam fillings are comparatively easy and fast to install inteeth and perform fairly well. Amalgam fillings, however, are quitebrittle and have a high incidence of fracture or splitting. About 25% ofthe dental amalgam fillings can be expected to fail and have to bereplaced. Since 1 12 million Americans have an average of 5.5 amalgamfillings, the total number of failures is large.

An examination of failed amalgam fillings has established that failureis due not only to compressive forces but also to tensile forces. Whilethe compressive strength of dental amalgam is quite high, the tensilestrength is only about one-tenth to one-sixth of the compressivestrength. The low tensile strength renders the amalgam inadequate towithstand many of the tensile forces applied to fillings. Accordingly,if the tensile strength of the amalgam can be increased, strongerfillings with less likelihood of failure can be made.

Young and Johnson, in the Journal of Dental Research, 47, 457 (1967)report mixing high purity tin and mercury, sealing the mixture into anevacuated quartz tube and heating the mixture to 300C. They reportincreased tensile strength as the mercury goes from 0 to and decreasedtensile strength going from 10 to mercury. The conclude that the way toimprove the tensile strength of amalgam may be in reducing the amount ofthe tin-mercury phase present.

When particulate dental alloy is manufactured, it acquires an oxidecoating which is believed to lower the tensile strength of amalgam madefrom it. However, the presence of the oxide coating slows the formationof a hard amalgam enough to give the dentist time to prepare it and filla tooth while the amalgam is still plastic. While use of an alloy freeof an oxide coating will give an amalgam of increased tensile strength,the dentist would find great difficulty in using it before it hardened.

According to the present invention, the tensile strength of a dentalamalgam can be significantly increased by mixing the powdered alloy andmercury in an inert atmosphere, such as an atmosphere of argon, heliumor neon or a mixture of such gases, to form the amalgam. With respect tothis invention, an inert gas is defined as any gas that does notchemically react with either the dental amalgam alloy or mercury eitherin bulk or in a surface reaction under conditions of normal ambienttemperatures and pressures in the absence of applied electrical andmagnetic fields. Although the reason for the increased tensile strengthof the amalgam is not fully understood, it is believed that the absenceof oxygen, and perhaps also water vapor during the mixing of the alloyand mercury leads largely to the increased tensile strength. The inertatmosphere does not have an adverse effect on other qualities of thealloy, mercury or resulting amalgam.

In producing an amalgam of increased strength according to theinvention, the dentist does not require special equipment or tools ornew techniques. He need only from the amalgam from a powdered alloywhich is bathed in an inert atmosphere when the mercury is mixed withit.

The filling materials used by a dentist in forming an amalgam aremarketed in both bulk form and in capsule dosage form. The invention isuseful in both forms to produce amalgam of increased tensile strength.

Dental supply manufacturing concerns market in bulk mercury and thepowdered alloy used in dental amalgam. Many dentists purchase such bulkmaterials and use them in measured amounts as needed to make a filling.Those dentists can readily adapt the invention to continued use of suchbulk materials. The dentist need only evacuate a capsule or othercontainer, fill it with an inert gas and then add and mix the powderedalloy and mercury. After mixing, the amalgam is ready for insertion in atooth.

To facilitate use of bulk powdered alloy by the dentist, the alloy canbe marketed in small glass bottles or cans containing an inert gas. Thecontainer and alloy can be purged of air, as by a vacuum, and the alloypackaged in the container in an inert atmosphere supplied by argon,helium or neon. After the container is opened the inert atmosphere islikely to be replaced with air unless precautions are taken. Therefore,if the alloy is not all to be used within a short time, it is advisablefor the open container to be stored in a bath of inert gas. A storagebox equipped with a fast opening cover and means to flood it with aninert gas can be used by the dentist for this purpose.

The capsule form in which the dental amalgam ingredients are alsosupplied to dentists lends itself more suitably to practice of theinvention than the bulk form of material. This is because the capsulesare sized to provide only enough amalgam for the fillings in progress.

Capsules of dental amalgam filling materials come in variousconstructions but they all apparently include a small container in theform of a capsule which holds a powdered alloy filling dosage and arupturable chamber holding liquid mercury. The ingredients arepremeasured (commonly designated as predosed in dental literature) sothat when totally mixed together an amalgam is obtained with the rightproportion of each ingredient. The amount of liquid mercury in thecapsule is generally about equal in weight to the amount of alloy. Bysuitable means, such as pressing or screwing down a cap or cover, orpuncturing the mercury-containing chamber in some other way such as witha pin, the mercury is released to flow into admixture with the powderedalloy without opening the capsule. The capsule is then generally placedin a shaking or mixing machine to complete the mixing. After thoroughmixing is obtained, the amalgam is removed from the capsule ready forcompaction in a tooth cavity. U.S.

Pat. Nos. 3,655,035; 3,655,037; 3,425,598 and 3,415,360 illustratecapsules which can be used for dental amalgam materials.

Filling of capsules containing the alloy powder and liquid mercury withan inert gas can be effected in a number of ways. One procedure is tofirst place the alloy powder in an open capsule. The capsule can then beplaced in a conventional dry box together with the capsule capcontaining the chamber or pouch of mercury and the dry box then pumpedto a high vacuum. The dry box then can be flooded with an inert gas.This will also cause each capsule to fill with the inert gas. The capcontaining the mercury can then be placed on the capsule. Suchmanipulations can be achieved by hand operations through glove-filledports in the dry box after it is filled with the inert gas. Once thecapsules are capped, the dry box can be opened and the capsules removed.The capsules can then be distributed to dentists for use by them inmaking amalgam fillings.

The inert gas-filled capsules are employed by mixing the mercury andpowdered alloy together before the capsule is opened. After the amalgamis produced the capsule is opened, the amalgam is removed and is placedin the cavity to be filled using conventional procedures.

The following examples are presented to further illustrate theinvention.

EXAMPLE 1 Ten amalgam test specimens were prepared according to theprocedures on pages 132 to 135 of the Guide to Dental Materials andDevices, referred to supra. Capsules containing premeasured about equalamounts by weight of mercury and alloy (commercially available asAmalcap 2, H. D. Justi Co.) were used. The product is included in the1970-71 List of Certified Dental Materials of the American DentalSociety. The alloy analyzed 72 wt silver, 26.8 wt tin, 2.05 wt mercury,263 ppm copper and 7.65 ppm zinc. Each specimen was prepared from thecontents of a single capsule. The caps were removed from the capsulesand the caps containing the mercury pouch as well as the capsule bodycontaining the powdered alloy were placed in a dry box. Air was removedfrom the capsules and alloy powder by evacuation of the dry box. The drybox was then filled with argon. While still in the argon atmosphere inthe dry box, the caps were then placed on the capsules and the assembledcapsules were then removed from the dry box. The mercury and powderedalloy were mixed together (triturated) in the argon atmosphere, for 5.5seconds, using a mechanical (Silamat) amalgamator oscillating atapproximately 4570 rpm. The amalgam was compacted in the prescribedmanner and allowed to age 24 hours. The specimens were then subjected tothe standard diametral compression test procedure in Guide to DentalMaterials and Devices supra, to determine the diametral tensile strengthof each specimen. The results are reported in Table l-Continued Spec-Load at Specimen Specimen Diametral imen fracture length diameterTensile Strength number (kg) (mm) (mm) Kg/mm Psi The mean diametraltensile strength was 6543.6 psi with a standard deviation of 927.2 psi.

The above procedure was repeated but the amalgam specimens were preparedfrom materials mixed in air. The results obtained are reported in Table2.

The mean diametral tensile strength was 5654.2 psi with a standarddeviation of 1071.9 psi.

EXAMPLE 2 Twenty test specimens were prepared following the sameprocedures (with the exception that the trituration time was 6 seconds)and using the same materials employed in Example 1. The specimens werethen subjected to the standard diametral compression test to determinethe diametral tensile strength of each specimen. The results arereported in Table 3.

Table 3 Load at Specimen Specimen Diametral Specimen Fracture lengthdiameter Tensile Str- Number* (lb) (in) (in) ength (psi) Specimen Nos.24 and 34 were rejected due to incomplete fracture.

The mean diametral tensile strength was 6603.2 psi with a standarddeviation of 1276.6 psi.

The above procedure was repeated but the amalgam specimens were preparedfrom materials mixed in air. The results are reported in Table 4.

Table 4 Load at Specimen Specimen Diametral Specimen Fracture Lengthdiameter Tensile Str- Number* (lb) (in) (in) ength (psi) "Specimen No.59 was rejected due to incomplete fracture.

The mean diametral tensile strength was 5949.7 psi with a standarddeviation of 1 143.8 psi.

EXAMPLE 3 Twenty amalgam test specimens were prepared according to theprocedures outlined below. The same commercial capsules containingpremeasured amounts of mercury and alloy as in Example 1 were used. Eachspecimen was prepared from the contents of a single capsule. The capswere removed from the capsules and the caps containing the mercury pouchas well as the capsule body containing the powdered alloy were placed ina dry box. Air was removed from the capsules and alloy powder byevacuation of the dry box. The dry box was then filled with argon. Whilestill in the argon atmosphere in the dry box, the caps were then placedon the capsules and the assembled capsules were then removed from thedry box. The mercury and powdered alloy were mixed together (triturated)in the argon atmosphere, for 6 seconds, using a mechanical (Silamat)amalgamator oscillating at approximately 4570 rpm. The amalgam wascompacted in the following manner.

Step 1 The amalgam was removed from the capsule and divided into fiveapproximately equal increments. Time allowed: 20 seconds.

Step 2 Using rubber tipped tweezers, an increment was placed into astandard die cavity and was compacted with one thrust, using a 3 mmdental condenser. Using a 1.5 mm dental condenser, the mass wascompacted with fifteen 5 lb thrusts. Time allowed: 25 seconds.

Steps 3 and 4 Step 2 was repeated. The amalgam from each step was placedin the die on top of the amalgam previously placed there.

Time allowed: 25 seconds for each Step.

Step 5 Excess mercury was removed with the condenser and a vacuum line.Time allowed: 10 seconds.

Steps 6 and 7 Repeat Steps 2 and 5. Time allowed: 35 seconds each forSteps 6 and 7.

Step 8 The top surface of the specimen was smoothed with one thrust ofthe 3 mm diameter dental condenser and the specimen ejected.

Time allowed: 5 seconds.

Step 9 At the end of one hour, the top end of the specimen was trimmedflat, with the use of a razor blade.

After ageing for 24 hours, each specimen was subjected to the standarddiametral compression test to determine its diametral tensile strength.The results are reported in Table 5.

Table 5 Load at Specimen Specimen Diametral Specimen Fracture lengthdiameter Tensile Str- Number* (lb) (in) (in) ength (psi) Specimen Nos.70 and 74 were rejected due to incomplete fracture.

Table 6 Load at Specimen Specimen Diametral Specimen Fracture lengthdiameter Tensile Str- Number (1b) (in) (in) ength (psi) The meandiametral tensile strength was 5828.6 psi with a standard deviation of1472.8 psi.

EXAMPLE 4 Two amalgam test specimens were prepared according to theprocedures outlined below. The same commercial capsules containingpremeasured amounts of mercury and alloy as in Example 1 were used. Eachspecimen was prepared from the contents of fourteen capsules. The capswere removed from the capsules and the caps containing the mercury pouchas well as the capsule body containing the powdered alloy were placed ina dry box. Air was removed from the capsules and alloy powder byevacuation of the dry box. The dry box was then filled with argon. Whilestill in the argon atmosphere in the dry box, the caps were then placedon the capsules and the assembled capsules were then removed from thedry box. The mercury and powdered alloy were mixed together (triturated)in the argon atmosphere, for 5.5 seconds, using a mechanical (Silamat)amalgamator oscillating at approximately 4570 rpm. The amalgam wascompacted in the following manner.

*Step 1 a. Mercury was introduced into the alloy in each of fourcapsules. b. Each capsule was then triturated in the described manner.c. The resulting amalgam from the four capsules was placed in a steeldie.

d. The amalgam was leveled and partially compacted along the sides andends of the die. e. The amalgam was compacted in a hydraulic press at ahydrostatic pressure of 5000 psi. The pressure was maintained for 15seconds. As mercury was expressed, additional hydraulic jack strokeswere required to maintain the correct pressure; five such strokes wereapplied during the 15 second interval. "The total allowable time betweenthe compaction of two consecutive layers was three minutes.

f. Excess mercury was removed with a cotton swab.

*Step 2 Step 1 was repeated. The amalgam from this step was placed onthe amalgam in the die from Step 1.

*Step 3 Step 1 was repeated using only three capsules. The amalgam fromthis step was placed on the amalgam in the die from the previous steps.

*Step 4 Step 3 was repeated.

Step 5 The specimen was allowed to cure 24 hours and then ejected fromthe die.

Step 6 The specimen was polished (using emery cloth) to the followingdimensions: 0.197 X 0.197 X 1.575 inches. Approximately 0.007 inch wasremoved from each cross-sectional direction.

Step 7 Using a mechanical grinder, a groove (test section) was machinedon opposing sides of the specimen in order to reduce the cross-sectionalarea to 0.025 sq. in.

*The total allowable time between the compaction of two consecutivelayers was three minutes.

Each specimen was epoxy cemented into tensile test specimen hold-downpads. The specimens were carefully aligned to minimize bending momentsduring the tensile test. A cross-head speed of 0.05 inch per minute wasused to determine the fracture load under uniaxial stress. The resultsare reported in Table 7.

Table 7 Specimen Uniaxial Tensile Strength Number (psi) The averageuniaxial tensile strength for these specimens was 8041 psi.

The above procedure was repeated but the amalgam specimens were preparedfrom the same materials mixed in air. The results are reported in Table8.

Table 8 Specimen Uniaxial Tensile Strength Number (psi) The averageuniaxial tensile strength of these specimens was 6129 psi.

SUMMARY OF DATA The implications of the data reported in Examples 1-4can be summarized as in Table 9.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. The method of making a dental amalgam which comprises combiningmercury and a particulate dental alloy, used in making dental amalgam,in an inert atmosphere.

2. The method of claim 1 in which the inert atmosphere comprises a gasthat does not chemically react with either dental amalgam alloy ormercury either in bulk or in a surface reaction under conditions ofnormal ambient temperatures or pressures in the absence of appliedelectrical and magnetic fields.

3. The method of claim 1 in which the inert gas is a member of the groupconsisting of argon, helium, neon and mixtures thereof.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3 90144 DATED 1 June 17, 1975 v (5) David A. Hansen It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 46, change "The" to They-; column 2, line 13, change"from" to form; column 6, line 46, before "Table 6'' insert (para.) Themean diametral tensile strength was 6411.5 psi with a standard deviationof 1455.9 psi. (para.) The above procedure was repeated but the amalgamspecimens were prepared from materials mixed in air. The results arereported in Table 6.--; column 8, line 65, change "1" to 2.

Signed and Scaled this twenty-third D ay Of December 1 9 75 [SEAL] 1tles t:

RUTH C. M A SON C. MARSHALL DANN Alrestmg Officer (ummissiuneroj'larenls and Trademarks

1. THE METHOD OF MAKING A DENTAL AMALGAM WHICH COMPRISES COMBININGMERCURY AND A PARTICULATE DENTAL ALLOY, USED IN MAKING DENTAL AMALGAM,IN AN INERT ATMOSPHERE.
 2. The method of claim 1 in which the inertatmosphere comprises a gas that does not chemically react with eitherdental amalgam alloy or mercury either in bulk or in a surface reactionunder conditions of normal ambient temperatures or pressures in theabsence of applied electrical and magnetic fields.
 3. The method ofclaim 1 in which the inert gas is a member of the group consisting ofargon, helium, neon and mixtures thereof.